1. Field of the Invention
Embodiments of the invention relate to the field of ion implantation for forming semiconductor structures. More particularly, the present invention relates to a method for controlling deflection of a charged particle beam within a graded electrostatic lens.
2. Discussion of Related Art
Ion implanters are widely used in semiconductor manufacturing to selectively alter conductivity of materials. In a typical ion implanter, ions generated from an ion source are directed through a series of beam-line components that may include one or more analyzing magnets and a plurality of electrodes. The analyzing magnets select desired ion species, filter out contaminant species and ions having undesirable energies, and adjust ion beam quality at a target wafer. Suitably shaped electrodes may modify the energy and the shape of an ion beam.
FIG. 1 shows a conventional ion implanter 100 which comprises an ion source 102, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet analyzer 110, and a second deceleration (D2) stage 112. The D1 and D2 deceleration stages (also known as “deceleration lenses”) are each comprised of multiple electrodes with a defined aperture to allow an ion beam to pass therethrough. By applying different combinations of voltage potentials to the multiple electrodes, the D1 and D2 deceleration lenses can manipulate ion energies and cause the ion beam to hit a target wafer at a desired energy.
The above-mentioned D1 or D2 deceleration lenses are typically electrostatic triode (or tetrode) deceleration lenses. FIG. 2 shows a perspective view of a conventional electrostatic triode deceleration lens 200. The electrostatic triode deceleration lens 200 comprises three sets of electrodes: entrance electrodes 202 (also referred to as “terminal electrodes”), suppression electrodes 204 (or “focusing electrodes”), and exit electrodes 206 (also referred to as “ground electrodes” though not necessarily connected to earth ground). A conventional electrostatic tetrode deceleration lens is similar to the electrostatic triode deceleration lens 200, except that a tetrode lens has an additional set of suppression electrodes (or focusing electrodes) between the suppression electrodes 204 and the exit electrodes 206. In the electrostatic triode deceleration lens 200, each set of electrodes may have a space/gap to allow an ion beam 20 to pass therethrough (e.g., in the +z direction along the beam direction). As shown in FIG. 2, each set of electrodes may include two conductive pieces electrically coupled to each other to share a same voltage potential. Alternatively, each set of electrodes may be a one-piece structure with an aperture for the ion beam 20 to pass therethrough. As such, each set of electrodes is effectively a single electrode having a single voltage potential. For simplicity, each set of electrodes are herein referred to in singular. That is, the entrance electrodes 202 are referred to as an “entrance electrode 202,” the suppression electrodes 204 are referred to as a “suppression electrode 204,” and the exit electrodes 206 are referred to as an “exit electrode 206.”
In operation, the entrance electrode 202, the suppression electrode 204, and the exit electrode 206 are independently biased such that the energy and/or shape of the ion beam 20 is manipulated in the following fashion. The ion beam 20 may enter the electrostatic triode deceleration lens 200 through the entrance electrode 202 and may have an initial energy of, for example, 10-20 keV. Ions in the ion beam 20 may be accelerated between the entrance electrode 202 and the suppression electrode 204. Upon reaching the suppression electrode 204, the ion beam 20 may have an energy of, for example, approximately 30 keV or higher. Between the suppression electrode 204 and the exit electrode 206, the ions in the ion beam 20 may be decelerated, typically to an energy that is closer to the one used for ion implantation of a target wafer. In one example, the ion beam 20 may have an energy of approximately 3-5 keV or lower when it exits the electrostatic triode deceleration lens 200.
The significant changes in ion energies that take place in the electrostatic triode deceleration lens 200 may have a substantial impact on a shape of the ion beam 20. For example, the deceleration lens 200, which may provide co-local deflection for filtering energetic neutrals, may face challenges associated with control of deflection angle and beam focus. Voltage needed to control deflection of the ion beam 20 may depend on the energy of the beam (e.g., both input and output), whereas voltage to control focus of the ion beam 20 may be varied to accommodate ion beams with different current and height. This may lead to difficulty in tuning the ion beam 20 since tuning the size of the ion beam 20 (focus) may not be readily feasible if a position of the ion beam 20 also continues to vary. Conventional systems and methods do not provide a solution for independently controlling the deflection and/or focus of an ion beam in a co-locally deflecting and decelerating lens.
Moreover, it may be desirable to focus high perveance beams in a deceleration lens system that simultaneously deflects the beam through a desired deflection angle. In order to obtain sufficient focusing for high perveance beams, using known deceleration lens systems it may be necessary to use electrode voltages that are impractically large and thus raise the risk of dielectric breakdown. Thus the performance may be limited by the maximum practical voltage that can be applied to the electrodes, thereby limiting the maximum strength of the fields that can be applied to the beam to values that are insufficient to perform the desirable focusing/deflection field strengths. In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion implantation technologies.